1. Field of the Invention
The present invention relates to the deposition of anti-wear coatings on titanium or titanium alloy parts, and the coated parts thus obtained.
2. Summary of the Prior Art
It is very difficult to obtain very adherent deposits on titanium and titanium alloy parts because of their high passivitiy.
It has already been proposed to subject such parts to be coated to a preliminary treatment before proceeding with the deposition of the actual anti-wear coating, so as to improve the adherence of the latter. Examples of the preliminary treatments which have been proposed are:
Anodic attack in mixtures of glycol--hydrofluoric acid, acetic acid;
Predeposition of zinc from glycol--metal fluoride mixtures, or aqueous mixtures based on fluoroboric, hydrofluoric acids and metal salts;
Long duration pickling in concentrated hydrochloric or sulphuric-hydrochloric acids, followed by deposition of iron, nickel or cobalt in a very acidic bath.
In all of these methods it is necessary or recommended to combine the preliminary treatment with a thermal treatment between 400.degree. C. and 800.degree. C. in an atmosphere which does not contaminate titanium, in order to improve the firmness of the metal coating. However, this firmness is never excellent. In particular, the coating obtained does not withstand machining or finish-grinding.
It has also been proposed in FR-A-1 322 970 to carry out a preliminary chemical treatment in an oxidixing medium, which comprises subjecting the part to the action of a bath of chromic anhydride, alkaline phosphate and hydrofluoric acid for from 5 to 30 minutes at a temperature between 35.degree. C. and 100.degree. C. However this treatment has the double drawback of generating hydrides in the coating and effecting an undesirable penetration of hydrogen into the substrate during subsequent electrolytic operations.
It is therefore an object of the invention to be free of the above-mentioned drawbacks by doing away with preliminary chemical treatments of the substrate, thereby avoiding the production of hydrides and curbing the penetration of hydrogen into the substrate during the electrolytic process of depositing the anti-wear coating.
It is also an object of the invention to enable an anti-wear coating to be deposited on titanium parts while reducing fatigue failure compared to the known processes, thus permitting the use of coated titanium substrates for parts subjected to cyclic fatigue, where such parts obtained by the known processes would not be acceptable.
A further object of the invention is to achieve a deposition sequence in which the technique of nickel deposition by magnetron cathodic spraying (cathode sputtering) so as to obtain a particularly adherent sub-layer on the substrate is associated with an electrolytic deposition permitting the deposition of a final anti-wear coating.
Yet another object of the invention is to define parameters for the deposition of nickel by cathodic spraying, compatible with subsequent electrolytic depositions.